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FIGURE 51.25
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Circuit geometry. (Courtesy of IPC.)
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51.11.2.5 PTH Imperfections. Nothing affects the function of the PCB more than imperfections in the PTH. These imperfections are usually not observable from the surface, and microsectional sampling of holes is limited to almost statistical insignificance. Despite this, evaluations of the microsection represent one of the most important evaluations that can be done on a PCB. 51.11.2.5.1 Annular Ring. Minimum internal annular ring is evaluated on the internal copper land that exhibits the shortest distance between its outer edge and where it ends in or at the plating in the hole. It should be noted that a microsection identifies layer shift in only one direction and that microsections in both x and y directions should be evaluated to determine accurately the actual shift that may be present. Horizontal sections from the top of the PCB down to the layer in question give a 360 view of the pad and can be used to exactly identify minimum annular ring and layer shift. The procurement documentation should specify the requirement for minimum annular ring (see Fig. 51.26), and in cases where the allowable annular ring is zero or breakout, the breakout cannot be allowed in the area where the circuit enters the pad, as this will reduce the circuit s current-carrying capacity. 51.11.2.5.2 Lifted Lands. Lifted lands on the PCB surface prior to thermal stress are not allowed and if found, the entire inspection lot should be reevaluated to the externally observable visual requirements for lifted lands described earlier in this chapter. Lifted lands after thermal stress are allowed. 51.11.2.5.3 Cracks. Cracks in the foils and platings in the microsection are broken into the following six categories (see Fig. 51.27):
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FIGURE 51.26 Internal annular ring measurement, horizontal and vertical microsection (does not include plating). (Courtesy of IPC.)
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A type cracks occur in the external foil. They do not extend into the plating and are acceptable for all classes of PCBs. B type cracks occur in the external foil.They go completely through the foil and extend into, but not completely through, the plating and are acceptable only for Class 1 PCBs. C type cracks occur in the internal foil and are nonconforming for all classes of PCBs. D type cracks completely penetrate the external foil and plating and are nonconforming for all classes of PCBs. E type cracks occur in the barrel plating of the hole. They are non-conforming for all classes of PCBs. F type cracks occur in the corner of the plating of the hole and are nonconforming for all classes of PCBs.
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FIGURE 51.27
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Types of cracks in the PTH, vertical microsection. (Courtesy of IPC.)
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51.11.2.5.4 Plating Nodules. Plating nodules should not reduce the PTH diameter below the minimum requirement specified in the procurement documentation. 51.11.2.5.5 Plating Thickness. Plating thickness should be measured at three places in the plated hole and averaged. The absolute minimum plating thickness observed should also be recorded.The minimum plating thickness requirements for the three-area average along with the absolute minimum thickness observed are defined in IPC-6012 (see Fig. 51.28).
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FIGURE 51.28 Measurement of plating thickness in PTH, vertical microsection. (Courtesy of Sandia Laboratories.)
51.11.2.5.6 Plating Voids. When found in the microsection, plating voids should be limited to one per microsection. No void should span more than 5 percent of the PCB thickness or be present at the interface of internal copper foil to the plating. 51.11.2.5.7 Solder Resist Thickness. When the procurement documentation specifies this thickness, it should be measured at the thinnest area observed between the copper and the surface of the solder resist. This is usually at the corner of the copper trace. 51.11.2.5.8 Wicking. During the drilling and hole cleaning process, glass fibers from the base material can be removed inward from the hole surface. During the copper-plating process, the copper will follow this glass removal topography, creating pockets of plating that extend laterally from the hole into the base material. This wicking can significantly reduce the electrical conductor spacing between holes or between holes and internal circuits. The maximum allowances for wicking can be found in IPC-A-600. (see Fig. 51.29). 51.11.2.5.9 Interconnection Separation. This is defined as the separation between the internal copper foil and the copper plating in the hole. It is also sometimes referred to as post separation.Any evidence of this imperfection in the FIGURE 51.29 Wicking, vermicrosection makes the associated PCBs nonconforming. This imperfection is a tical microsection (Courtesy of IPC.) severe issue that will likely reduce the service life of the PCB (see Fig. 51.30).
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